Piezoelectric ceramic-polymer composites, hereafter referred to as piezoelectric composites and piezoelectric ceramics, are used in transducers for medical ultrasound imaging. In medical ultrasound there is an increasing need to improve the imaging range and resolution, as determined by the sensitivity and bandwidth of the transducer. There is also a move toward high frequency transducer use for endoscopic surgical procedures such as laparoscopy and intravascular imaging. Thus, piezoelectric composite transducers provide these medical procedures with better axial and lateral resolution.
Three major advantages enjoyed by piezoelectric composites over piezoelectric ceramics are reduced specific acoustic impedance (Z), increased thickness coupling (k.sub.t), and reduced planar coupling (k.sub.p). Trade offs in piezoelectric composite design must be made, as these three parameters cannot be optimized simultaneously.
Presently, piezoelectric composites having 1-3 connectivity are most commonly used in medical ultrasound transducer applications. 1-3 connectivity composites are commonly used because of the significantly reduced planar coupling constants (k.sub.p) that can be achieved over those that can be achieved by using homogeneous ceramic or isotropic 3-3 connectivity composites of the same materials.
A 1-3 connectivity composite is one where one phase, typically the ceramic phase, is self connected in one direction (z direction or thickness direction) of the composite, while the other phase, typically the infiltrate phase, is self connected in three directions of the composite. A 1-3 connectivity composite may be made by embedding a phase of aligned ceramic fibers or rods in an infiltrate phase or may be made by cutting deep grooves in a monolithic block of ceramic and filling the empty spaces with an infiltrate phase. The latter technique is referred to as dice and fill. In a 1-3 connectivity composite, the minimum dimensions of the ceramic pillars and the spaces are limited by a dicing blade and the mechanical strength of the ceramic. As a result, 1-3 composites do not always satisfy the increasing need for high frequency transducer operation in medical ultrasound applications.
It is desirable to have a piezoelectric composite with a structure of an interconnected lamelli and an interconnected interlamellar region which can be processed into an anisotropic 3-3 connectivity composite having improved electromechanical properties of a 1-3 connectivity composite. It is also desirable to have a piezoelectric composite that can be used in an electromechanical device having reduced planar coupling, a high thickness coupling constant, and reduced specific acoustic impedance. It is further desirable to have a composite and a resulting electromechanical device that is cost effective, reproducible, and adaptable into a manufacturing environment.